Conventional crescent internal gear pumps typically include rotatably driven, intermeshing ring and pinion gears that are disposed in an eccentric relationship within a cylindrical gear housing. The ring gear, pinion gear, and the housing are sandwiched between a front cover and an end cover. A crescent is disposed radially intermediate the pinion gear and the ring gear. During operation of the pump, the ring and pinion gears are rotatably driven, and fluid from a fluid inlet in the gear housing is entrained within expanding gaps between the teeth of the ring and pinion gears and the crescent. As the ring and pinion gears continue to rotate, the gaps shrink and the entrained fluid is forced to exit the gear housing through a fluid outlet.
A disadvantage that is commonly associated with crescent internal gear pumps of the type described above is that the efficiency of such a pump is highly dependent on the precision of clearances between the components of the pump. For example, pump efficiency is influenced by the sizes of clearances between the faces of the ring and pinion gears and the faces of the front and end covers, and also by the presence and size of gaps between the end of the crescent and the front cover. Ideally, no gap would exist between the end of the crescent and front cover.
In common practice, the tight tolerances that are required in conventional crescent internal gear pumps are achieved using precise machining or even manual hand lapping. This drives manufacturing to use very expensive machines and machining techniques. Often, it also requires that components be sorted in a time-consuming, laborious manner in order to identify combinations of components that achieve desired relative clearances. Still further, individual components must generally be held to tolerances in excess of what is required for a particular component in order to account for tolerance stack-up when the components are assembled.
In view of the foregoing, it would be advantageous to provide an efficient crescent internal gear pump that can be manufactured without applying strict tolerances to individual components of the pump.